The process of pressure swing adsorption (PSA) to enrich the concentration of a gas, such as the oxygen concentration, is known in the prior art. The challenge has been to generate sufficient quantities or flow rates of the enriched gas, in a sufficiently high concentration, to enable the therapeutic use of a PSA machine. A further challenge has been to make such a machine portable, that is relatively small and lightweight, self-sufficient, and self contained for realistic portability by a patient over an extended period of time.
It is also known in the prior art to attempt to conserve the target gas using an oxygen conserving device (OCD), that is, conserve the use of for example high concentration oxygen by a patient so as to pro-long the supply of the target gas from. Thus in one aspect of the present invention, it may be advantageous to combine a target gas concentrator, for example, an oxygen concentrator, with an OCD to enhance the duration of supply of the enriched gas from the gas concentrator, thereby enhancing the self sufficiency of both the device and patient.
As known in the prior art, oxygen is prescribed for patients suffering from chronic obstructive pulmonary disorder (COPD). By way of background, the practice of providing oxygen to COPD patients is known as long-term oxygen therapy (LTOT). The oxygen cylinders conventionally provided to patients for therapeutic use are typically large and heavy. Smaller, lighter cylinders are available but have a limited duration of oxygen flow.
Oxygen conserving devices were introduced which enabled cylinders to deliver sufficient oxygen for greater periods of time by only providing oxygen when the patient would inspire. In those devices, a fixed volume of oxygen is delivered, at a high flow rate, to the patient upon detection of inspiration. The volume of oxygen delivered per breath to the patient, per setting, is claimed to produce the same blood oxygen concentration in the patient as a continuous flow device at the same setting.
An oxygen-conserving ratio can be defined based on the volume delivered to the patient by a continuous flow oxygen system, at any given setting, compared to the volume delivered by the OCD. There are two common fixed ratios, 3:1 and 6:1. A doctor may find that a particular patient at rest will have sufficiently high blood oxygen concentration for a given pulse dose as metered by the OCD at a particular setting. However, if the patients' oxygenation requirements increase as a consequence of physical activity, increasing the amount of oxygen metered based on a fixed ratio pulse dose may not be effective. It could be that the patient requires more oxygen than the conserving device will supply to maintain blood oxygenation. To compensate for this a doctor may choose to prescribe the highest setting. This of course would then not be the most efficient use of oxygen.
The patient inspiration is detected by the OCD when a partial vacuum is produced in the nasal cannula. If breathing occurs partially through the mouth, such as when a patient is sleeping, the vacuum in the nasal cavity may be too shallow to be detected by the sensor. Increasing the sensitivity of the sensor introduces a new problem of possible false inspiration detection whenever the cannula is bumped, as a result of the differential pressure created within the cannula as it shakes.
In the field of gas concentrators it is known to use zeolite to adsorb nitrogen in an oxygen concentrator. The use of zeolite herein is intended to be exemplary. It would be known to one skilled in the art to tailor the use of a specific adsorbent, whether a particular type of zeolite or other adsorbent. As is known in the prior art, zeolite consists of molecular sized polyhedral cages. Oxygen and nitrogen molecules (for example) can access the inside of these cages through holes in the crystalline structure. The crystalline structure contains cations. Gas adsorption occurs when molecules are attached to these cations through electrostatic forces. Nitrogen molecules bind stronger to the zeolite cations than oxygen molecules. As a result, if a mixture of nitrogen and oxygen, such as found in atmospheric air, is pressurized into a chamber full of zeolite particles, nitrogen will adsorb into the zeolite particles more readily than oxygen. There will be a higher concentration of oxygen in the empty space between the zeolite particles, (hereinafter referred to as zeolite void space), than there was in the original gas mixture.
A conventional pressure swing adsorption gas separation cycle in an oxygen concentrator works as follows:    (a) A first cylindrical zeolite filled chamber is pressurized from the feed end with atmospheric air, while oxygen enriched gas exits from the product end through a gas flow restrictor.    (b) The oxygen enriched gas flow passes through a conduit junction. A portion of the oxygen-enriched gas is delivered for end use. The remaining portion of the oxygen-enriched gas travels through the product end of a second cylindrical zeolite chamber and is vented to atmosphere from a vent located at the feed end of the chamber. This gas flow is counter current to the direction of pressurization of the second chamber, (hereinafter referred to as counter flow), to push nitrogen out of the second chamber.    (c) The second zeolite filled chamber is pressurized from the feed end with atmospheric air, while oxygen enriched gas exits from the product end through a gas flow restrictor.    (d) The oxygen enriched gas flow passes through the conduit junction. A portion of the oxygen-enriched gas is delivered for end use. The remaining portion of the oxygen enriched gas travels through the product end of the first chamber and is vented to atmosphere from a vent located at the feed end of the chamber. This gas flow is counter current to the direction of pressurization of the first chamber to push nitrogen out of the first chamber.The cycle is then repeated.
During oxygen enriched gas generation nitrogen is left adsorbed into the zeolite particle structure. A conventional approach to removing the nitrogen from the chamber is to blow oxygen enriched gas across the zeolite from the product end of the chamber to the feed end of the chamber. This counter-flow pushes nitrogen gas, as a wave, to the feed end of the zeolite chamber and out the vent to the atmosphere. Since nitrogen is strongly bound to the zeolite cations, it takes a large quantity of oxygen enriched gas flow in conjunction with depressurization to remove it. Another method to remove the nitrogen from the zeolite chamber is to evacuate the chamber with a vacuum pump.
Conventional PSA sieve bed design thus provides for the flow of gas through a sieve bed from the inlet or feed end to the outlet or product end. The sieve beds are completely filled with molecular sieve. A calibrated orifice at the product end provides resistance to the flow of gas through the sieve bed. This resistance to flow provides the necessary pressurization of the sieve bed to facilitate nitrogen adsorption by the molecular sieve. The bed is pressurized for a period of time, which corresponds to the propagation of the mass transfer zone through the bed. The mass transfer zone is a build up of nitrogen, which moves as a front from the feed end of the bed to the product end of the bed. The mass transfer zone moves as a consequence of gas flow through the bed, but may propagate independently of the gas flow. The point at which the mass transfer zone reaches the outlet (product end), where any further pressurization will result in high concentrations of nitrogen leaving the product end, is referred to as “breakthrough”.
By way of analogy, when humid air is introduced into one end of a dry desiccant filled cylinder, the desiccant incrementally soaks up the water vapor in the air. As a result, the gas that has passed through the desiccant bed is dryer than the gas that entered it. Water is adsorbed first by the first available desiccant particles that appear in the flow stream. Thus the desiccant near the entrance of the desiccant filled cylinder will be filled with water vapor well before the desiccant near the exit of the cylinder. As the total volume of humid air that has passed through the desiccant increases, the volume of the desiccant filled with water increases from the feed end of the cylinder to the exit of the cylinder. Eventually the desiccant that is near the exit of the cylinder is filled with water as well. At this point, no more water vapor can be adsorbed. If more water vapor is added after this point, the water vapor will just pass straight through the desiccant filled cylinder. This is an example of “breakthrough”. Likewise, for any given pressure, a molecular sieve material such as zeolite can only adsorb a finite volume of nitrogen before breakthrough occurs.
Although applicants do not wish to be held to any particular theory of operation of a device according to the present invention, in applicant's opinion in considering the adsorption performance of a sieve bed design, there may be several key dynamics which function interdependent of each other. The critical performance parameters (excluding the bed design itself and related flow dynamics) may be: working pressure of the bed, rate to pressurization, restrictive orifice size, duration of pressurization, flow rate through the sieve bed and molecular sieve performance. If any one of these parameters in a current sieve bed design is changed without adjusting the others, the propagation of the mass transfer zone or breakthrough may be affected.
For instance: If the rate to pressurization is too slow, diffusion may occur in the bed and concentration purity may not be reached. If the rate to pressurization is too fast, “jetting” may occur and cause shadow zones (inactive areas in the sieve bed), early breakthrough, and compromise concentration purity. If the orifice size is wrong, all flow dynamics may be affected which may result in diffusion or early breakthrough. If the duration of pressurization is too short the bed may not be as efficient as it was designed to be and produce less gas. If the duration of pressurization is too long, breakthrough may occur and compromise concentration purity. If the flow rate through the sieve bed is too fast, diffusion of the mass transfer zone or early breakthrough may occur. If the flow rate through the sieve bed is too slow diffusion of the mass transfer zone could occur. If the bulk-loading ratio changes, that is, if the potential for quantity of gas to be adsorbed by the molecular sieve changes, concentration purity may be affected. If the adsorbent selectivity changes, that is, if the preference for the target gas to be adsorbed over the product gas by the molecular sieve changes, concentration purity may be affected.